Normal hemostasis depends on the presence of arachidonic acid (AA) in the phospholipids of platelets. AA, which is released from the phospholipids, is converted into the endoperoxides and thromboxane A2. These compounds are potent vasoconstrictors and powerful inducers of platelet aggregation and, thus, important regulators of the hemostatic process. Abnormally high or low levels of AA in the platelet phospholipids may lead to thrombotic disorders or to bleeding tendencies, respectively, and little is known about the mechanisms which control the amounts of AA in the phospholipids of platelets. This knowledge is essential for the development of rationales to prevent these pathologic states. A worthwhile approach to the prevention of thrombosis is manipulation of the diet to control the ratio in platelet phospholipids of AA to other prostaglandin precursors whose metabolites inhibit platelet aggregation and are anti-thrombotic in nature. This proposal is concerned with an understanding of the mechanisms which regulate the amounts of AA and other unsaturated fatty acids in human platelet phospholipids. The specific aims of this proposal are to study in the subcellular fractions of human platelets: 1) the role of the remodelling pathway of phospholipid biosynthesis in controlling the unsaturated fatty acid composition of platelet phospholipids, 2) the role of the de novo pathway of phospholipid biosynthesis in controlling the unsaturated fatty acid composition of platelet phospholipids, and 3) the role of fatty acyl CoA synthetase in controlling the metabolism of unsaturated fatty acids. Activation of fatty acids to the CoA ester by fatty acyl CoA synthetase is a prerequisite for their further fatty acid metabolism and incorporation into phospholipids. The selectivity of fatty acyl group incorporation into phospholipids by the remodelling or de novo pathways will depend both on the composition of the pool of fatty CoA esters and on the specificity of the fatty acyl CoA transferases for the fatty acy CoA's and the lysophospholipid or glycerol-3-phosphate acceptors.